1. Field of the Invention
The present invention relates to a speed measuring apparatus using the principle of a spatial filter, for example, to a speed measuring apparatus used as a non-contact speed meter carried in a vehicle.
2. Description of the Prior Art
FIG. 5 shows the construction of a conventional spatial filter type speed measuring apparatus (see "Electronics", Ohm Publishing Co., Ltd., January, 1984, pp. 73 to 76).
The spatial filter type speed measuring apparatus has a light projecting unit (LPU) 1 and a light receiving unit (LRU) 2 which are fixed to a holder 10. This is mounted on an automobile or the like with the side on which light is emitted from the light projecting unit 1 and the side on which light impinges on the light receiving unit 2 being directed toward a road surface L.
The light projecting unit 1 has a concave reflecting mirror 13, a lamp (light source) 14 and an aspherical lens 15 contained in a case 11. It is so adapted that light emitted from the lamp 14 is both directly projected through the aspherical lens 15, and reflected from the concave reflecting mirror 13 and projected through the aspherical lens 15, onto the road surface L.
The light receiving unit 2 has an objective lens 27, a slit 28, a differential type spatial filter detector 22 and a differential amplifier 23 contained in a case 21. It is so adapted that light reflected from the road surface L converges on the the objective lens 27, and light passed through the slit 28 forms an image on the detector 22.
The differential type spatial filter detector 22 comprises a semiconductor substrate 30 and two comb-shaped photodiodes 31 and 32 formed on the substrate 30, as shown in FIG. 6. The photodiodes 31 and 32 are insulated from each other. The teeth or fingers of the comb-shaped photodiode 31 are alternately arranged with the teeth or fingers of the comb-shaped photodiode 32. Let P be the pitch between the teeth or fingers.
The difference between the level of an output signal of the photodiode 31 and the level of an output signal of the photodiode 32 is calculated by the differential amplifier 23. Let f be the frequency of an output signal of the differential amplifier 23, and K be the coefficient related to the image-forming magnification in an optical system of the light receiving unit 2. In this case, the speed V of a vehicle is generally given by the following equation: EQU V=K.multidot.P.multidot.f.
The road surface or the like includes grains of various sizes from one millimeter to several centimeters such as small stones, sand and asphalt and irregular patterns due to tire tracks, causing irregularities in reflection (color irregularities, surface irregularities or the like). The spatial filter type speed measuring apparatus is constructed so as to take a component of the signal due to the irregularities in reflection spaced at .DELTA. (=about 2.3 millimeters) from the irregular patterns. More specifically, the output signal of the differential amplifier 23 turns into a pulse train by waveform shaping after passing through a band-pass filter. The pulses are counted by a counter. Since the pulses occur due to the irregularities in reflection spaced at .DELTA., the speed V can be found by multiplying a counted value of pulses corresponding to one second by the value .DELTA..
When the spatial filter type speed measuring apparatus of the above described construction is used as, for example, a speed meter carried in a vehicle while facing a road surface, however, there is a problem.
More specifically, if there is a puddle or the like on the road surface, most of light emitted from the light projecting unit is specularly reflected from the surface of the puddle and the light specularly reflected forms an image on the light receiving unit. Accordingly, the irregularities in reflection from the road surface at the bottom of the puddle cannot be detected, thereby making it impossible to measure the exact speed. Under bad conditions such as rainy weather, it is particularly difficult to measure the speed.